In recent years, motors driven by the inverter of PWM (Pulse Width Modulation) method have prevailed in the market. In the case of the motors driven by the PWM inverters, an electric potential at the neutral point of the winding cannot be 0 (zero), so that an electric potential difference (hereinafter referred to as an axial voltage) is generated between the outer ring and the inner ring of the bearing. The axial voltage contains a high-frequency component produced by a switching operation. When the axial voltage reaches a dielectric breakdown voltage of the oil film inside the bearing, a micro electric current runs in the bearing, thereby inviting an electric erosion in the bearing. A progress of the electric erosion will result in wavy abrasion on the inner ring, the outer ring, or the balls of the bearing, and the wavy abrasion sometimes incurs an abnormal sound. The electric erosion is thus one of chief factors causing defects of the motor.
The drive circuit (including a control circuit and others) used in the inverter for driving the motor by the PWM method includes a power supply circuit, which is electrically insulated from both of the primary side circuit thereof and a grounding of the primary side circuit.
The following measures have been taken for preventing the electric erosion:
(1) The inner ring and the outer ring of the bearing are made conductive with each other.
(2) The inner ring is insulated from the outer ring of the bearing.
(3) The axial voltage is lowered.
Method (1) employs, e.g. a conductive lubricant for the bearing. However, the conductive lubricant lowers the conductivity with the lapse of time, and is short of reliability in slide action. A brush can be mounted on a rotary shaft for making the inner ring and the outer ring conductive; however, this method incurs abrasion dust of the brush and requires a space for the brush.
Method (2) employs, e.g. non-conductive ceramic balls inside the bearing instead of iron balls; however, this method is expensive although a high anti-erosion effect can be expected. This method thus cannot be used for general-purpose motors.
Method (3), e.g. electrically shorts a stator iron-core to a conductive metallic bracket, thereby varying an electrostatic capacity for lowering the axial voltage. This method is a public domain and disclosed in, e.g. Patent Literature 1.
Various art has been disclosed for suppressing the electric erosion of the bearing of the motor, for instance, a structure in which a stator iron-core of the motor is grounded.
Patent Literature 1 discloses that the stator iron-core is electrically shorted to the bracket for lowering impedance of the stator, thereby preventing the electric erosion on the bearing.
To be more specific, the motors used in devices operated in a wet area, e.g. washing machine and dish washer, have a risk of inviting an electric shock, so that not only an insulation on a charging section (primary insulation) but also an independent insulation (hereinafter referred to as an additional insulation) is needed. On the other hand, the other motors used in an indoor unit or an outdoor unit of the air-conditioner, a water heater, or an air-cleaner have no risk of the electric shock, so that the additional insulation is not needed. The motors to be used in the indoor unit or outdoor unit of the air-conditioner, the water heater, or the air-cleaner thus include the rotors not insulated, so that the impedance on the rotor side (inner ring side of the bearing) stays low, while the impedance on the stator side (outer ring side of the bearing) stays high. In this case, a difference in voltage drop is produced, namely, the electric potential on the inner ring side is high while that on the outer ring side is low, so that they fall into an unbalanced state of the impedance, and such a high axial voltage may invite an electric erosion to the bearing.
In order to avoid the foregoing problem, Patent Literature 1 discloses a method of lowering the impedance on the stator side (outer ring side) for the impedance to approximate an impedance on the rotor side (inner ring side). This method can be achieved by electrically shorting the iron core of the stator to the bracket.
In recent years, a mold motor has been proposed for improving the reliability, to be more specific, a stationary member such as an iron core of a stator is molded of a mold-material. The bearing is then fixed with such an insulating mold-material instead of the metallic bracket for suppressing an unneeded high-frequency electric current produced on the outer ring side or an unneeded high frequency electric current flowing between the inner ring and the outer ring of the bearing. However, such mold-material is made of resin, so that its strength is not enough for fixing the bearing, and resin-mold cannot expect highly accurate dimensions. As a result, malfunctions caused by creep tend to occur in the bearing. To be more specific, in a case where a space is available between the outer ring of the bearing and the inner wall of the housing, transfer load produces force on the shaft in the radial direction. This force induces a sliding phenomenon due to a relative difference along the radial direction, and the sliding phenomenon is called “creep”. The “creep” can be suppressed, in general, by fixing the outer ring firmly to the housing, e.g. bracket. In recent years, the motor has been expected to output a greater power, so that the bearing needs to be fixed more firmly. To meet these needs, it is essential to take measures against the creep, e.g. a metal bracket made of steel plate excellent in dimensional accuracy is employed for fixing the bearing. The rotary shaft is, in general, supported by the bearings at two places, and the two bearings are preferably fixed with the metal brackets because of the strength as discussed above and an ease of implementation.
The conventional method disclosed in Patent Literature 1; however, encounters the following problem: This conventional method uses a shorting, so that it is impossible to adjust the impedance, and a structure or magnet material of the rotor sometimes raises the axial voltage. Here is another problem, i.e. since this method aims at lowering the impedance, the inner ring and the outer ring of the bearing are always kept balance with respect to the impedance in a state of high electric potential. In such a state, if the impedance loses the balance due to a working condition of the motor or due to dispersion in accuracy of assembling the stator with the rotor, the axial voltage rises opposing to the aim, so that the electric erosion may tend to occur.
Use of the conductive bracket on the counter output shaft side may cause the impedance to be lower than that in the case of using a bracket made of insulating resin. In other words, the housing made of resin exerts powerful insulating performance, so that no electric current flows between the inner ring and the outer ring of the bearing; however, the use of the conductive bracket lowers the insulating performance, so that an electric current flows between the inner ring and the outer ring. The counter output shaft side thus also tends to invite an electric erosion.    Patent Literature 1: Unexamined Japanese Patent Application Publication No. 2007-159302